Stereophonic amplifier circuit providing improved low frequency response



3,162,730 ING G. E. OWEN, JR

FIER CIRCUIT PROVID FREQUENCY RESPQNSE STEREOPHONIC AMPLI IMPROVED LOWOriginal Filed Aug. 1l. 1959 Dec. 22, 1964 fgwf, M

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United States Patent O 3,162,730 STEREPHNEC Ali/@MEER CIRQUE QRGVBRTGWRVEB LUV! FlVtEQlUENCY ihtltNSlE George Edwin Gwen, En, Melrose Park,lili., assigner to Motorola, inc., Chicago, lli., a corporation ofillinois Continuation of application Ser. No. tl, Aug. 1i, i959. rlfhisapplication Dec. Ztl, wel, Ser. No. 151,345 5 Claims. (Cl. fwn-ledit)This invention relates `generally to stereophonic signaltranslatingapparatus and more particularly to an improved system for enhancing basstones in such apparatus. This is a continuation of my copendingapplication Serial No. 833,098, filed August ll, 1959.

In order to provide inexpensive stereophonic sound reproductionequipment such as disk phonograph record players and the like, it isnecessary to reduce both the number and size of components and tosimplify the circuitry. For example, in the more expensive stereophonicequipment, separate large speakers, commonly known as woofers, arefurnished in addition to the minimum of a separate speaker for eachchannel to reproduce the low frequencies. Such woofers are expensive andrequire more power amplifying units to drive them, and therefore theyare often dispensed with in low cost stereophonic units. If costreduction can be accomplished without sacrificing the frequency rangeand faithfulness of response of such equipment, the more desirable itwill be. It is well known, however, that fewer and less expensivecomponents will ordinarily reduce the breadth of the reproducedfrequency band, especially at the lower end of the frequency scale. Inspite of this fact, it is still possible to achieve satisfactorystereophonic sound effects with such equipment since practicalexperimentation has shown that the bass frequencies (below about 250cycles) do not in themselves contribute significantly to thestereophonic effect. lt is sufficient that the equipment be capable ofreproducing frequencies generally in the range of 250 to 1500 cyclessince this general range of frequencies best promotes the stereo effect.Of course frequencies above and below this range are also important forobtaining overall high fidelity or faithful reproduction.

Accordingly, it is an object of the present invention to provide new andimproved stereophonic audio signaltranslating apparatus Vwhich is low incost yet wherein the low-frequency response may be substantiallyimproved over that ordinarily found in an inexpensive system.

Another object is to provide improved stereophonic amplifying apparatusin which the low frequency amplication may be adjustably boosted withoutthe addition of separate speakers or expensive components espe ciallydesigned for reproduction of low audio frequencies.

A feature of the present invention is the provision in multiple channelstereophonic amplifying apparatus of a low pass filter coupled betweenthe output of an amplifying `stage Ifor one of the channels and an inputcircuit of an amplifying stage for another channel to increase the basssignal level of the system.

Another feature is the provision in dual channel stereophonic apparatushaving an amplifying system for each channel of a variable frequencyselective network nonregeneratively coupled between an output circuit ofone of the amplifying system and an input circuit of the otheramplifying system to provide a controllable increase in the overall lowfrequency response of the apparatus.

In the accompanying drawing FIGURE 1 is a schematic diagram whichillustrates a bass boost circuit arrangement in accordance with theinvention.

In brief, a system is provided for boosting the bass l E,ld2,73dPatented Dec. 22, i964 "ice output of one amplifier in multiple channelstereophonic amplifrying apparatus to improve the low frequency responseof the entire apparatus. In a dual channel system, this is achieved -bycoupling a shunt circuit between an output circuit in the first channelamplifier and a common reference potential for the left and rightchannel signals. One leg of the shunt circuit includes a low passcapactive network for bypassing the medium and high frequency componentsof the first channel signal to the reference potential. These componentsmay be of the order of ZOO-300 cycles per second and above. This networkmay include a parallel connected variable resistor so that the bassboost effect can be controlled to suit the listener. Another leg of theshunt circuit includes a resistive network for coupling the selectedbass signals from the first channel amplifier to the second channelamplifier. An appropriate phased input circuit of the second channelamplifier is connected to this resistive leg, and usually at a matchedamplitude point therealong, so that the substantially in-phase firstchannel bass signals boost the bass signals being applied to the secondchannel amplifier.

Referring now to FIG. l, a conventional dual channel stereophonicphonograph is illustrated which has an amplifier L and associatedspeaker for reproducing left channel audio intelligence and anotheramplifier R and associated speaker for reproducing right channel audiointelligence. rThe right and left7 signals are derived from a disc typephonograph record 10, such as the socalled 45/45 stereophonic record nowon the market, by a dual channel audio signal source such as thestereophonic pickup cartridge 1l. A left channel output lead therefromsupplies left channel audio intelligence to the left channel input leadlf2 of amplifier L. Right channel audio intelligence is fed from theright channel output lead of cartridge 11 to the right channel inputlead i3 of amplifier R. Both input sockets containing leads 12 and i3are grounded for alternating current by the common capacitor 14connected between the respective socket shields and a common referencepotential. The outer surfaces of the respective cartridge jacks providethe return terminals to complete the cartridge output circuit.

v in the respective input circuits of the phonograph ampliiiers, apotentiometer 16 is connected across the output terminals of the leftchannel audio signal source while another potentiometer f7 is connectedacross the output terminals of the right channel audio signal source.The variable taps i9 and 21 of potentiometers 1o and 17 are ganged foroperation as a single volume control to tap off input signals developedacross the respective potentiometers. Variable tap 19 is connectedthrough a capacitor 22 to the control grid 23 of triode 2d, here shownas half of a double triode. Triode 2dis connected as a voltage amplifierstage 26, grid 23 being returned to the reference potential through agrid-leak resistor Z7. The cathode 23 of triode 23 is connected to thereference potential through a cathode bias resistor 29 and the anode 3iis connected through a load resistor 3?; and isolating resistor 33 to asource of direct current indicated as B+. The left channel amplifiedaudio signals are developed across load resistor 32, and the junction ofresistors 32 and 33 is bypassed to the reference potential for thesesignals by capacitor 34.

The output of voltage amplifier stage 26 is coupled in a conventionalmanner to the control grid 35 of a pentode 3f through coupling capacitor37 and suppressor resistor 38. Pentode 36 is connected as a poweramplifier stage 37 with the control grid 35 being returned to thereference potential through one side of an in-comrnon connected balancepotentiometer 3S, each side thereof serving as an adjustable grid-leal;resistor. Suppressor grid 39 and cathode 49 of the pentode 36 areconnected to the reference potential through a cathode bias resistor 42and bypass capacit-or 43, resistor 42 and capacitor 43 also beingconnected in-common between the two amplifiers to thereby serve the samefunction in the circuit of amplifier R. The screen grid 44 of pentode 36is connected to the source of B+ by the screen resistor 33 and isbypassed to the reference potential by the capacitor 34, both also beingin-common connected between the left and right amplifier circuits.

The anode 46 of tube 36 is connected through the primary winding ofoutput transformer 47 to the source of B+ potential and is then bypassedto the reference potential by capacitor 48 connected to the B+ source.To complete the left channel system, the secondary winding of outputtransformer 47 is connected to drive the coil of speaker 49 wherein theleft channel signal is converted to audible sound waves.

A small portion of the amplifier energy in the plate circuit ofamplifier stage 37 is fed back to the cathode circuit of the precedingamplifier stage 26 to provide negative feedback to improve frequencyresponse and to cancel out any distortion generated in stage 3'7. Thisis accomplished in a conventional manner by connecting a shunt circuit,consisting of a resistor 51, a capacitor 52, another resistor 53 andbypass capacitor 34 all in series, between the junction of plate 46 withthe primary transformer 47 and the reference potential. The cathode 2Sof tube 24 is then connected to the junction of resistor 53 andcapacitor 52 so that feedback voltage in phase opposition is applied tothe input circuit of amplifier stage 26.

It will be evident from the schematic diagram of FIG. l that the rightchannel amplifier R is a duplicate of amplifier L, the two amplifierscontaining symmetrical circuitry and being mounted on the same chassis.

The operation of the symmetrical amplifiers L and R is entirelyconventional and will be readily understood from the foregoingdescription by those skilled vin Vthe art. Briefly, and withoutconsidering for the moment further structure of the invention to besubsequently described, an audio signal having frequency componentsextending over a given audio range and characteristic of the left sideof a sound source is supplied from the left channel lead of astereophonic sound source, such as the cartridge 11, and is impressedacross the input circuit of amplifier stage 26 of amplifier L. Theamplifier stages 26 and 37 of amplifier L are designed to amplify in astandard inanner substantially all of the frequency components of theaudio range. The amplifier audio signal is developed across the primarywinding of output transformer 47 to induce a signal in the secondarywinding which is then reproduced by the speaker 49. The operation of theright channel amplifier R with respect to the right channel intelligenceapplied thereto is the same as the left channel amplifier L.

As an additional feature of the particular phonograph described, theleft channel amplifier is provided with a tone control circuit includinga potentiometer 54 having a variable tap 56 connected to the referencepotential. One end of potentiometer 54 is connected through frequencyselective capacitor 57 to ithe anode 31 of tube 24 while the other endof the potentiometer is connected through resistor 58 to a fixed tapprovided on potentiometer 16. The portion of the potentiometer 54between variable tap 56 and resistor 58 is shunted by capacitor 59.Frequency response of the left channel system may be adjusted by movingthe tap 56. Movement to the left will increase the treble response ofstagey 26, since an increasing amount of resistance is placed in serieswith capacitor 57 while the effect of capacitor 59 in providing bassresponse for this stage is decreased. The variable tap 56 ofpotentiometer S4 is ganged with an identical tone control circuitconnected across a voltage amplification stage 61 in amplifier R forsimultaneous tone control of both channels.

As was previously noted, the circuitry of the right channel system issymmetrical with the circuitry of the left channel system, permittingsome of the components to be connected in common between the systems.Thus the balance control potentiometer 3S functions as a grid-leakresistor in each of the input circuits of the power amplification stages37 and 62 of the respective left and right channel amplifiers. By movingthe variable tap of this potentiometer, a simultaneous and inversechange in the input voltages applied to the respective amplificationstages may be produced to compensate for any signal imbalanced betweenthe input circuits.

A controlled bass boost is achieved by adding further circuitry to theabove described stereophonic sound reproduction system. ln the disclosedembodiment of the invention this is accomplished by connecting one sideof a blocking capacitor 61 to the output circuit of power amplificationstage 37 at the junction of plate 46 and the primary winding oftransformer 47, the other side of capacitor 61 being series connected toresistor 62. Resistor 62 in turn is connected to the reference potentialthrough two shunt circuits. The first shunt circuit consists of alow-pass filter network including a capacitor 63 parallel connected witha variable resistor 64, the latter being manually adjustable by means oftap 66. The second shunt circuit contains a coupling resistor 67connected between the junction of the variable low-pass filter networkwith resistor 62 and the tap which defines a portion 13 of potentiometer17.

In operation, the bass boost circuit applies a portion of thelow-frequency components in the amplified left channel signal from anoutput circuit of one of the amplifier stages in amplifier L to an inputcircuit of one of the amplifier stages in amplifier R. When theconnection between the amplifiers is made from the output circuit of thefinal amplifying stage, a signal of relatively large amplitude isavailable. As previously described, such a signal, having left channelfrequency `components extending over a given audio range, is developedacross the primary winding of the output transformer 47. This signal iscoupled to the bass boost circuit by capacitor 61, which has a largevalue of capacitance so as not to eliminate the low frequency componentsof the signal. Capacitor 6l also isolates or blocks the passage ofdirect current from the output circuit to the bass boost circuit. Inorder to limit the amount of signal current in the bass boost circuit toa small proportion of that through the primary of the output transformer47, a relatively large resistor 62 is series connected to capacitor 61.Thus, even when the subsequent portions of the bass boost circuit arecompletely bypassed by moving the variable tap 66 all the way to theright on resistor 64, only a small portion of the output current will owthrough capacitor 61 and resistor 62.

Considering now the operation of the remaining pol'- tions of the bassboost circuit, when the variable tap 66 is in the far right position theentire range of component frequencies coupled through capacitor 61 fromthe left channel output will be non-selectively shunted through tap 66to the reference potential. This setting of the tap will thus correspondto the minimum effect position of the bass boost control. As the tap ismoved to the left more resistance is added in series with resistor 62and in parallel with capacitor 63. Since the reactance of capacitor 63varies inversely with frequency while resistor 64 remains essentiallynon-selective to the band of frequencies, most of the middle and highfrequency components will be bypassed to the reference potential by thecapacitive leg of the circuit. The value of capacitance of capacitor 63will determine the lower cut-off characteristic for these frequencycomponents, which generally are on the order of 200-300 cycles persecond and above. The voltage drop for the relatively low frequencycomponents, generally those of 20G-300 cycles and below, will now bedivided between resistor 64 and resistor 62. Thus, the increase in thevoltage drop across resistor 64 will be confined primarily to the lowfrequency or bass components in the audio range. These components arethen developed over a voltage divider network consisting of the seriesresistor 67 and potentiometer portion 1S connected between the junctionof resistor 62 with resistor 64 and the reference potential. Since theinput circuit of voltage amplifier stage 61 in amplifier R is connectedacross potentiometer 17 which includes the portion 18, the low frequencysignals thus selected from amplifier L will be applied to the input ofamplifier R.

The amplitude range of the left channel low frequency components appliedto the input of stage 61 is determined by the resistances of resistornetworks 62, 64, 67 and 1S. For maximum bass boost a typical designwould limit the amplitude of these frequencies to about the same orderof amplitude found in the right channel low frequency components derivedfrom the cartridge 11 and developed across the input of stage 61 orother input circuits selected for the connection. Resistor 67 alsoserves to substantially isolate the input signal of stage 61 fromcapacitor 63 to prevent distortion of the right channel middle andhighfrequency components.

In the disclosed embodiment of the invention it has been found thatcomponents of the following types and values provide satisfactoryresults:

T riode tubes in stages 26 and 61 Type 12AX7A. Pentode tubes in stages37 and 62 Type 35C5. Capacitor 61 .0l microfarad. Resistor 62 400,000ohms. Capacitor 63 .001 microfarad. Resistor 64 1 megohm. Resistor 67100,000 ohms. Potentiometer 17 l megohrn. Resistance of portion 1S ofpotentiometer 17 330,000 ohms.

It is to be understood that the above tube types and circuit componentvalues are not intended to limit the invention as variations therefromwill occur to those skilled in the art without departing from theinventive concepts disclosed by the above described structure.

It is to be noted that the left channel low-frequency components in theoutput of the second stage of amplifier L and the right channellow-frequency components applied to the input circuit of the first stagein amplifier R will be essentially in phase, and therefor no noticeablesubtraction or distortion will occur between these mixed signals. Thisis due to the fact that in stereophonic reproduct-ion phase andamplitude differences do not become significant until the middle andrelatively high frequency audio signals are reached. Thus, astereophonic phonograph may be provided with an adjustable bass boostcircuit in accordance with the invention without deteriorating thelow-frequency response of amplifier R.

Since the invention is primarily intended for addition to inexpensivestereophonic reproducing systems, a marked increase in the faithfulnessof reproduction will be noted as more of the left channel bass signal isapplied to a right channel input circuit. As was explained previously,the less expensive the system, the narrower the range of undistortedfrequency reproduction. By adding a predetermined portion of theamplified left channel bass signals, correctly phased, to the rightchannel signal applied to a selected input circuit of amplifier R, anincreased amount of bass will be developed in this applied signal. Theseboosted bass frequencies will then be amplified in amplifier R toproduce an output which is actually distorted in favor of the lowfrequencies on the right side of the phonograph. However, due to thenondirectional characteristics of the low frequencies, the listenerwillbe unable to detect this intentional amplitude imbalance between theleft and right systems and the overall effect will be stereophonic soundreproduction of higher fidelity.

The attenuation of the low frequency components in both the left andright systems caused by sacrificing the more expensive components inorder to maintain low cost will thus be effectively compensated for bythe circuit arrangement of the invention. To produce an equivalentoutput having the same frequency response without improving thecomponent quality of the set would require another stage ofamplification in one of the systems. Thus the cost of a tube andattendant components is saved by the simple circuitry of the invention.

From the foregoing description it will be apparent that similar resultscould be obtained from the above circuitry or its equivalent by couplingan appropriate output stage of the right channel system to anappropriate input stage of the left channel system without therebydeviating from the inventive concept of the disclosed embodiment.

i claim: A

l. Stereophonic signal reproducing apparatus including in combination,first and second amplifying means having respective input and outputcircuits, means for applying stereophonic signals respectively to saidinput circuits of said first and second amplifying means, first andsecond sound reproducing means directly connected to said outputcircuits of said first and second amplifying means respectively foraudibly reproducing the stereophonic signals, low pass lter meanscoupled from said output circuit of said rst amplifying means to saidinput circuit of said second amplifying means, thereby increasing thelow frequency output level of said apparatus beyond the reproductionprovided by the frequency response characteristic of said first and`second amplifying means.

2. Stereophonic signal reproducing apparatus including in combination,first and second amplifying means having substantially the samefrequency response characteristic, said amplifying means havingrespective input and output circuits, means for applying stereophonicsignals respectively to said input circuits of said first and secondamplifying means, first and second sound reproducing means directlyconnected respectively to said output circuits of said first and secondamplifying means for audibly reproducing the stereophonic signals, lowpass lter means coupled from said output circuit of said firstamplifying means to said input circuit of said second amplifying meansthereby increasing the low frequency output level of said apparatusbeyond the reproduction provided by the frequency responsecharacteristic of said first and second amplifying means.

3. Stereophonic signal reproducing apparatus including in combination, amultiple channel audio signal source, first and secondV amplifying meansconnected to said source, rst and second sound reproducing meansdirectly connected to said first and second amplifying meansrespectively for audibly reproducing the stereophonic signals, avariable low pass filter network coupled from a point between said firstamplifying means and said first sound reproducing means to a pointbetween said source and said second amplifying means so that a portionof a low frequency signal from said source is amplified by both saidfirst and said second amplifying means to increase the low frequencyoutput level of said apparatus beyond the reproduction provided by thefrequency response characteristic of said first and second amplifyingmeans.

4. Stereophonic signal reproducing apparatus including in combination, adual channel audio signal source, first and second amplifying meansconnected to said source, first and second sound reproducing meansdirectly connected to said first and second amplifying meansrespectively for audibly reproducing the stereophonic signals, avariable low pass filter coupled from a point between said firstamplifying means and said sound reproducing means to a point betweensaid source and said second amplifying means, said filter havingvariable impedance means and capacitance means coupled in parallel, sothat a variable low frequency signal from said source may be amplifiedby both said first and second amplifying means to increase the lowfrequency output level of said apparatus beyond the reproductionprovided by the frequency response characteristic of said rst and secondamplifying means, said variable impedance means forming an adjustablecontrol for determining the amount of increase in the low frequencyoutput level of said apparatus.

5. Stereophonic reproducing apparatus including in combination, firstand second driver amplifier devices having substantially the samefrequency response and having respective input and output circuits,phonograph pickup means for applying stereophonic signals respectivelyto said input circuits of said first and second driver amplifierdevices, first and second power amplifier devices having substantiallythe same frequency response and having respective input and outputcircuits, said output circuits of said first and second driver amplifierdevices being directly connected to said input circuits of said firstand second power amplifier devices respectively, first and second soundreproducing means directly connected to said output circuits of saidfirst and second power amplifier devices respectively for audiblyreproducing the stereophonic signals, a variable low pass filter coupledfrom the output circuits of said first power amplifier device to theinput circuit of said second driver amplifier device, said filter havingvariable impedance means and capacitance means coupled in parallel sothat a variable low frequency signal from said source may be amplifiedby all of said devices to increase the low frequency output level ofsaid apparatus beyond the reproduction provided by the frequencyresponse characteristics of said devices, said variable impedance meansforming an adjustable control for determining the amount of increase inlthe low frequency output level of said apparatus.

References Cited in the file of this patent UNITED STATES PATENTS2,876,299 Robins Mar. 3, 1959 FOREIGN PATENTS 1,051,525 Germany Feb. 26,1959

1. STEROPHONIC SIGNAL REPRODUCING APPARATUS INCLUDING IN COMBINATION,FIRST AND SECOND AMPLIFYING MEANS HAVING RESPECTIVE INPUT AND OUTPUTCIRCUITS, MEANS FOR APPLYING STEROPHONIC SIGNALS RESPECTIVELY TO SAIDINPUT CIRCUITS OF SAID FIRST AND SECOND AMPLIFYING MEANS, FIRST ANDSECOND SOUND REPRODUCING MEANS DIRECTLY CONNECTED TO SAID OUTPUTCIRCUITS OF SAID FIRST AND SECOND AMPLIFYING MEANS RESPECTIVELY FORAUDIBLY REPRODUCING THE STEROPHONIC SIGNALS, LOW PASS FILTER MEANSCOUPLED FROM SAID OUTPUT CIRCUIT FOR SAID FIRST AMPLIFYING MEANS TO SAIDINPUT CIRCUIT OF SAID SECOND AMPLIFYING MEANS, THEREBY INCREASING THELOW FREQUENCY OUTPUT LEVEL OF SAID APPARATUS BEYOND THE RE-